1. Field of the Invention
This invention relates to a blade support limb, and particularly to a blade support limb for a vertical axis wind turbine.
2. Description of the Related Art
Vertical axis wind turbines are widely used for generating power and each comprises a wind rotor. However, there is a non-neglectable problem with the existing vertical axis wind turbines: as an ambient wind speed is greater than a rated wind speed, a power output of a wind rotor correspondingly increases, as the ambient wind speed is far greater than the rated wind speed, the wind rotor is incapable of operating. FIG. 1 shows the structure of the blade support limb in the prior art. The wind mill of the vertical axis wind turbine adopts straight airfoil blades (6). The blades are parallel to the vertical shaft (7). The blades are usually fixed on one end of the blade support limbs (1) that rotate around the vertical shaft (7). When the wind blows on the blades at certain angles, the lift and drag are produced on the blades. When the lift exceeds the drag, a moment of rotation around the vertical shaft is produced on the blades to drive the wind mill to rotate. FIG. 2 shows the installation method for blades in the prior art. The line between the leading edge and the trailing edge of a blade is called the “chord,” and the angle formed between blade chord and the tangent is called the “blade rotation angle,” which can be represented by a. When the blade chord is parallel to the tangent, a is zero degree. When the blade rotates clockwise around the vertical shaft, a is positive, and when the blade rotates counter-clockwise around the vertical shaft, a is negative. The blades are usually fixed on one end of blade support limbs (or cantilever supports or support limbs) or ring supports and usually can't rotate relative to the support limbs, i.e., the blade rotation angle α is fixed. One end of the blade support limb is connected to the blade, and the other end is connected to the vertical shaft.
Normally, the blade is installed at a cantilever support limb (or cantilever support or blade support limb) or ring support and cannot rotate relative to the blade support limb, i.e., a fixed support limb structure with constant blade rotation angle α, one end of the blade support limb is connected with the blade, and other end of it is connected with the vertical axis.
For the effect of blade rotation angle α on the rotation speed of the wind rotor of vertical axis wind turbine, please refer to Chinese Patent Application Publication No. CN1831330A and Chinese Patent Application No. 200610028267.7 by the same inventor.
When the blade rotation angle α is small, the wind rotor can get a relatively high rotation speed and wind energy utilization ratio. For a vertical axis wind turbine with constant blade rotating angle α, the rotation speed of its wind rotor is proportional to the environmental wind speed. Under a constant blade rotation angle α, the higher the environmental wind speed, the higher the rotation speed of wind rotor. The wind energy is proportional to the cube of the environmental wind speed. When the environmental wind speed increases from 10 m/s to 25 m/s, the wind energy increases by nearly 16 times. Therefore, the vertical axis wind turbine needs to have a motor with relatively high power to match with the wind rotor so that it can maintain operation under a relatively high environmental wind speed. And thus the designed rated wind speed of the vertical axis wind turbine must be very high. However, considering the characteristics of a motor, if the rated wind speed is high, the power will be very low under a low wind speed. And there are mostly low wind speeds under the natural state, which thus decreases the efficiency of the vertical axis wind turbine. To enable the vertical axis wind turbine to maintain the rated output power within a relatively wind range of wind speeds, the wind rotor must still maintain a relatively constant rotation speed and output power after the environmental wind speed exceeds the rated wind speed so as to enable the vertical axis wind turbine to maintain operation under a relatively wide range of wind speeds and thus to enhance the value of vertical axis wind turbine.
Existing schemes overcoming such technical defects are to increase load resistance for small wind turbine with hundreds watts, apply “yawing and adjusting the pitch of blades” to large horizontal axis wind turbine and change angle of attack for large vertical axis wind turbine. Please refer to the Chinese Patent Application Publication No. CN1831330A and Chinese Patent Application No. 200610028267.7 by the same inventor. However, considering the costs, such schemes are not suitable for the vertical axis wind turbine from several kilowatts to dozens of kilowatts output.